Suggestion: Add `is_output_op_return` function

Currently the way of doing this is:
```
fn ensure_output_is_op_return(index: u32) {
    match jet::output_null_datum(index, 0) {
        Some(entry: Option<Either<(u2, u256), Either<u1, u4>>>) => (),
        None => panic!(),
    }
}
```

It might be easier to have a few functions around this function